Blood clotting is an important mechanism in preventing blood loss in response to blood vessel injury. Sometimes, however, clotting occurs in the blood vessels of a healthy person in a process called thrombosis. The resulting blood clot, or thrombus, is largely composed of blood cell fragments known as platelets or thrombocytes. A thrombus that dislodges and circulates within the blood (“embolizes”) is known as an embolus.
Thrombosis and embolism are associated with many cardiovascular diseases. When thrombosis blocks the normal flow of blood in arteries, a decreased supply of oxygen and glucose can cause tissue damage. The restricted blood flow, or ischemia, can cause injury to any organ and may even result in death. When a thrombus disturbs the supply of blood to the brain, a stroke may result. When a thrombus obstructs a coronary artery, the result can be a heart attack. Other diseases associated with thrombosis include angina pectoris, transient ischemic attack, peripheral arterial disease (PAD), peripheral vascular disease (PVD) and arterial thrombosis, to name a few. Accordingly, there is medical interest in treating thrombosis.
Thrombosis is stimulated by the arachidonic acid-derived prostanoid thromboxane (TX) A2. TXA2 triggers platelet activation and aggregation by agonistically binding to receptors on the surface of platelets and stimulating the expression of integrins on the platelet surface. Integrins on one platelet are then bound by fibrinogen to other platelets, thereby building up a clot. TXA2 also stimulates contraction of various types of smooth muscle including vascular smooth muscle, leading to vasoconstriction, as well as of renal and pulmonary smooth muscle.
Attempts to treat thrombosis have involved targeting the synthesis of TXA2. An enzyme called cyclooxygenase (COX) produces prostaglandin (PG) H2 through its enzymatic conversion from the 20 carbon lipid arachidonic acid to generate a series of lipid mediators referred to as the prostanoids. In this synthetic pathway, the COX-derived PGH2 endoperoxide product is converted by a host of specific PG synthases to make the prostaglandins PGD2, PGE2, PGF2α and PGI2 (Prostacyclin) and by TXA synthase to make TXA2. The prostanoids are made in a cell- or tissue-specific manner and mediate a diverse range of physiologic roles in the body. By way of example, TXA2 is predominantly made in platelets and in activated macrophages. Thus, inhibiting COX, such as within platelets or macrophage, should reduce or prevent the synthesis of TXA2.
Non-steroidal anti-inflammatory drugs (NSAIDs), such as aspirin, inhibit COX, thus interfering with the synthesis of TXA2. However, since COX produces the other physiologically important prostanoids, NSAIDs can cause a general imbalance in prostanoid levels. This imbalance can actually increase the risk of thrombosis, leading to stroke and other problems. Furthermore, a large part of the population exhibits aspirin resistance. Also problematic, COX inhibitors are associated with the irritation of gastric mucosa, peptic ulceration, and renal failure.
Of note was the discovery that the COX enzyme exists as two distinct types or isoenzymes referred to as COX-1 and COX-2. Since COX-1 inhibition was thought to cause gastric irritation, selective inhibitors of COX-2 (coxibs) were developed. However, the coxibs have not proven satisfactory. For instance, coxibs appear to increase the risk of atherothrombosis and myocardial infarction, even with short-term use. Notably, the coxib Vioxx was withdrawn from the market after its use was shown to be associated with adverse thromboembolic events.
Given these problems with COX inhibitors, there is clinical interest in blocking the function of TXA2 by blocking the TXA2 receptor (the T prostanoid receptor, or in short the TP) at the platelet surface. A compound that binds to the TP antagonistically should inhibit TXA2 binding and platelet aggregation and thus thrombosis. Furthermore, as the primary COX-1/COX-2 product PGH2, an endoperoxide, also binds and activates the TP, antagonists of the TP should also impair its activation by PGH2. Moreover, in addition to its enzymatic conversion into the prostanoids through the COX-1/COX-2 catalyzed reactions, arachidonic acid can also be converted non-enzymatically into the isoprostanes through free-radical mechanisms. Noteworthy, the isoprostane 8-iso-PGF2α is the most abundant isoprostane generated during oxidative injury and actually mediates its actions/signals through the TP.
Hence, selective TP antagonists will have the added advantage over COX-1/COX-2 inhibitors, such as aspirin or coxibs, in that they will also inhibit the adverse actions of the isoprostane 8-iso-PGF2α generated during oxidative injury and of the endoperoxide PGH2, in addition to inhibiting the action of TXA2 itself. Unfortunately, existing TP antagonists have proven problematic. For example, they lack efficacy, TP specificity and target other receptors, such as the PGD2, platelet activating factor 4, or Leukotriene D4 receptors.
In humans and primates, but not in other species, TXA2 actually signals through two distinct TP receptor isoforms referred to as TPα and TPβ which are encoded by the same gene and differ exclusively in their distal carboxy-terminal primary amino acid sequences. Furthermore, the current TP antagonists do not discriminate between the two TPα and TPβ receptor isoforms which play similar, but not identical, roles. TPα, for example, is subject to desensitization in ways that TPβ is not and vice versa. Due to their distinct roles, in addition to developing general TP antagonists, there may also be clinical interest in compounds that can selectively interact with one or both isoforms of the TP.